When the United States was preparing to enter World War I, it was desperately short of material, jumping from a 30,000-man army to a projected 1-million-man force. Colt and other contractors could not fill the need for the issue handgun — the 1911 .45 Government Model. Both Colt and Smith & Wesson had production capacity geared up. Since the .45 automatic cartridge does not have a cartridge case rim, the ejector star could not eject the cartridge. S&W developed a thin sheet-metal clip to take the place of the cartridge case rim, neatly solving the problem of using the cartridge in a revolver. The moon clip was born, providing the necessary .05-inch headspace. There was now a single cartridge for both the automatic and the revolver in the supply chain.
This is a companion discussion topic for the original entry at https://www.usconcealedcarry.com/blog/moon-clips/